The Impacts of Bycatch from the Atlantic Tropical Tuna Purse Seine Fishery on Ecosystem Structure and Function

Global catches of tuna have increased steadily in the past several decades and this increase was largely driven by the switch to purse seine gears. Purse seine vessels target tropical tuna species in all the world’s oceans and have two main modes of fishing, catching schools of tuna associated with a floating object (FAD sets) or schools of unassociated tunas (free sets). Both modes of fishing strategies have the potential to catch unintended species, termed bycatch, however larger rates of bycatch are seen from FAD sets. The total amounts of bycatch caught by both fishing strategies can be difficult to quantify without 100% coverage of onboard observers recording bycatch. This dissertation presents methods to determine total bycatch from unobserved sets using variations of the stratified ratio estimator method. Bycatch is retained onboard the fishing vessel or discarded at sea, with the aim to discard the bycatch species alive, however there is the potential for high levels of post-release mortality. Impaired reflexes of species that are discarded at sea were measured and were a significant predictor of delayed mortality. Proportions of impaired reflexes and delayed mortality were paired with physiological indicators of stress in teleost species. The data obtained from the first part of the dissertation was used to build an Ecopath model of the Gulf of Guinea to understand the total impacts removals of the targeted tuna and bycatch species have on the ecosystem. This model was calibrated using available time series for the Gulf of Guinea. The fitted model was then used to build several scenarios with varying both bycatch removals and purse seine fishing effort. The largest changes to the ecosystem resulted from changes to the purse seine fishing effort, rather than the treatment of the bycatch.